ABSTRACT/SUMMARY Immunological homeostasis reflects a balance between the host response and the antigenic environment. In mucosal immunology, inflammatory bowel diseases (IBD), including ulcerative colitis or Crohn?s disease, reflect a disruption in homeostasis with exaggerated host responses to the local microbiota in genetically susceptible hosts. Our microbial communities are dynamic so regulatory Th cells induced in the periphery (pTreg) are important to maintain a flexible homeostasis with these diverse organisms. Many factors modify the metabolic balance that maintains homeostasis and Treg function. Relevant to this proposal, studies have associated a disruption in adenosine metabolism with IBD in humans and in animal models. Adenosine is a purine metabolite derived from ATP through its conversion to ADP and 5?AMP by CD39 while CD73 continues the metabolism to adenosine. As the production of ATP from dead cells or bacteria is pro-inflammatory, its catabolism to adenosine is one means to restrict inflammation. Adenosine has direct anti-inflammatory properties mediated primarily through the A2A adenosine receptor (A2AAR) expressed by lymphocytes as well as antigen presenting cells and innate lymphoid cells. Moreover, we present new findings suggesting that adenosine shifts the energy metabolism in Th cells in order to confer its anti-inflammatory effects. Other data show that the absence of adenosine initiates the expansion of pathogenic Th cells, a decrease in Treg, and selects for microbiota that transmit susceptibility to colitis. The hypothesis for this study is that adenosine is required to maintain immunological homeostasis in the digestive tract. More specifically, disrupting adenosine production or responsiveness impacts lymphoid cell fate that subsequently changes bacterial colonization, creates a dysbiosis and promotes inflammation. The broad objective of the project is to define the role of purine metabolism on the control of immunological homeostasis in the gut as addressed in the following interrelated Specific Aims: Aim 1: Identify how lymphoid-microbial homeostasis relies on adenosine. Aim 2: Determine how adenosine controls protective responses to microbiota Aim 3: Define the role of purine metabolism in controlling lymphoid cell fate. . The proposed experiments will explore novel aspects of immunological homeostasis. These studies will have a positive impact on the basic understanding of lymphoid cell plasticity and provide new knowledge that can be used to expedite the identification and development of therapeutic strategies for immune-mediated diseases.